Air and water heating and conditioning appliances utilizing gas combustion are in extensive usage worldwide, and their safe installation and use in occupied facilities is of ongoing concern. This concern has led, for example, to an entire industry based on sensing the presence of carbon monoxide in the occupied portions of such facilities. Such sensors are responsive to dangerous conditions only after the conditions are present in a facility and a threat to occupants, are of debatable sensitivity and reliability, are not remediative, and are thus less than adequate solutions to the safety problem.
A goal of some in the industry has been to provide means making facility/home HVAC safety sensor based rather than containment based (through the use of air ducts and the like). Modern combustion appliance installation must assure proper flue chimney draft, particularly in view of the large pressure differentials which power fans used for air distribution can produce. Presently, this is accomplished using air duct systems to isolate the air distribution system from the combustion air supply and flue venting. One consequence of this isolation is that the distribution air flow is stifled, impacting system efficiency (high distribution air flow is of key importance to heating and cooling energy efficiency). To replace or eliminate certain duct systems (particularly supply/return air duct systems) thereby to enhance distribution air flow, a sensor would be required that is responsive to low flue/chimney draft type pressure differentials and capable of differentiating flue draft airflow direction. To date, such a sensor with sufficiently high sensitivity and reliability has not existed.
Various approaches to sensing available flue draft in a variety of implementations have heretofore been suggested and or utilized (see U.S. Pat. Nos. 4,406,396 and 5,039,006). While useful, such approaches have required draft sensing at each combustion appliance and have not been simple to implement, install and/or adapt in various applications.
Improvements directed to alleviation of the lack of adequate distribution air flow in homes and other facilities, and thus improvement of heating/cooling efficiency, would be desirable. To convey one BTU of heat with 1° F. temperature rise in sea level air requires the movement of 55 cubic feet of air. A typical central furnace burning one therm (100,000 BTU) per hour requires air with a 30° F. temperature rise in the amount of 3000 cubic feet per minute (cfm). It is important to keep the temperature rise somewhat reasonable or the losses in the ducts due to thermal conduction and small air leaks will be substantial. An air conditioner (AC) or heat pump creating cooling or heating of 30,000 BTU per hour requires air with a 10° F. temperature rise in the amount of 2750 cfm. With this equipment, the figure of merit (heat removed divided by the net work input) is inversely proportional to the temperature difference between the source and the sink (indoors and outdoors). It is therefore very important to keep that temperature difference small or the distribution air temperature above or below the ambient small. These large airflow requirements are seldom met (even in modern homes the typical total airflow is down from these numbers by a factor of 5 to 10).
Under current building codes, the needed airflow requires either very large ducts or very streamlined high velocity ducts, both of which are expensive to provide and install and consume valuable building space. One solution to both problems would be ductless return air systems. Use of such an open return would greatly facilitate the utilization of low grade heat and cooling sources and the redistribution of air throughout the facility/home. However, to accommodate usage of such ductless return systems the open return air path (particularly at the combustion appliance) must be made safe. One solution would be to provide pressure sensing between the indoor and outdoor wherein pressure differentials as little as 0.005 inches of water could be sensed in the vicinity of the flue. But, as noted heretofore, no such sensing solution has been forthcoming, and suitable pressure sensors (having adequate sensitivity) have not been available. While a number of systems have been suggested which might be adapted and/or implemented (see, for example, U.S. Pat. Nos. 4,637,253, 6,328,647 and 6,983,652), none resolve all the problems which need to be addressed and/or satisfy the particular requirements of the industry. Further improvement could, therefore, still be utilized.